two gpu's, how to differentiate them in system settings pull down menu?
I think this is a trial and error kind of question.
Choose one of the GPUs on the list:



And do a render test.
Using an external application to monitor the GPU usage you can determine which card is being used to render.



In my case the one on the bottom of the list in blender is GPU1, but it might be different on your system.

Now, I would recommend enabling both cards to render... Even with the load of the display, it might cut your render time almost in half...